Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

150
FXUS63 KLOT 271142
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
542 AM CST Sat Dec 27 2025

.KEY MESSAGES...

- Areas of dense fog possible this morning, mainly near/west of
  the I-39 corridor.

- Showers and some thunderstorms likely areawide Sunday into
  early Sunday evening.

- Thunderstorms may pose a locally damaging wind threat Sunday
  afternoon/early evening, mainly southeast of I-55.

- Very strong winds will usher in drastically colder air Sunday
  night into Monday, with some snow showers possible.

&&

.DISCUSSION...
Issued at 332 AM CST Sat Dec 27 2025

Ceilings and visibilities have been very slow to come down so
far this morning, leading to some growing doubts about the
extent of the dense fog threat this morning. Ceilings have
fallen below 500ft far western and southwestern CWA and these
areas stand the best chances at seeing gradually stratus build
down to dense fog this morning. Interestingly, visibilities have
trended upward over the past couple of hours near the
Mississippi River and with light easterly low level flow
developing on the backside of the departing sfc ridge. I am
having some doubts about the dense fog threat into our area this
morning. Opted to maintain areas of dense fog in the grids after
11z or 12z, mainly near/west of the I-39 corridor as well as
near and south of highway 24.

GOES imagery early this morning depicts a massive stratus deck
blanketing the western Great Lakes and much of the Midwest.
Given the sharp inversion that this stratus is trapped beneath,
it seems likely that today will remain overcast. Given the
expected cloud cover have lowered high temps a couple more
degrees with low to mid 40s probably the best we`ll be able to
do today. Wouldn`t even be surprising to see some interior
portions of northern IL struggle to even reach 40F.

Tonight, a strong upper trough will emerge out onto the northern
and central High Plains. In response, surface cyclogenesis will
take place over Kansas with warm frontogenesis taking place
east from this developing cyclone across northern Missouri into
downstate IL and IN. As height falls spread well east of the
upper trough, it should encourage the development of a 40kt
925mb southerly low level jet across the Mississippi River
Valley. Strengthening isentropic ascent north of the developing
warm front and near the terminus of the low level jet should
result in blossoming area showers overnight tonight across our
CWA.

Strong low level theta-e advection and isentropic ascent is
likely to continue into Sunday morning north of the slowly
northward advancing warm front. Ascent will further be enhanced
by the increasingly diffluent/divergent flow in advance of the
approaching 140kt+ upper level jet. The strong and deep ascent,
couple with ample slant-wise and perhaps some weak upright
instability could allow for some embedded thunderstorms Sunday
morning. This strong forcing and convection will have
essentially record high PWATs leading to the threat of some
locally heavy rainfall Sunday morning into the early afternoon.
Particularly with any training heavier rainfall, which is a bit
of a concern given the low level baroclinic zone becoming
oriented parallel to the strengthening mid-upper level flow in
advance of the mid-upper level trough. Antecedent fairly low
stream flow and still largely unfrozen soils should help
mitigate the flooding risk, but infiltration rates tend to be
lower this time of year and with the potential for 0.75-1.50"
rainfall totals in 3 hours or less, hard to rule out some
nuisance type flooding of low lying and flood prone areas.

This morning rain and convection will probably slow the
northward advance of the warm front and associated unseasonably
warm and humid air mass. Eventually, the strong dynamics
associated with the deepening cyclone should drag the front
northward into at least our southern most CWA. Sfc dewpoints
south of the warm front near or just over 60F should at the very
least weaken the stability of the boundary layer if not result
in a weakly convectively unstable boundary layer conditions.
Wind profiles will only grow increasingly impressive with time
Sunday afternoon and evening as the cyclone enters a period of
rapid intensification. A strongly forced band of low topped
convection is likely to develop along the quickly advancing cold
front Sunday afternoon and evening. Given the strong forcing and
very impressive wind fields, it wouldn`t take much, even weak
convection`s downdrafts could potentially transport some of the
stronger flow just off the deck to the sfc and result in strong
to locally damaging wind gusts.

Weak low and mid level lapse rates will likely limit
destabilization, leaving the question of whether 60F dewpoints
in and of themselves will be sufficient for some weak boundary
layer destabilization. Just a few hundred J/kg of SB CAPE would
be sufficient for a more meaningful severe weather threat given
the strong forcing and wind fields. The difference between 300
J/kg of SBCAPE and no SBCAPE is really in the noise level of
model guidance at this distance and very difficult to predict.
Unfortunately, that very small difference in instability could
mean the difference between a gusty line of heavier showers and
a more intense squall line with locally damaging winds and
perhaps even a QLCS tornado threat given the extremely strong
low level shear. Interestingly, this time of year it is FAR
easier to get lightning with elevated convection in WAA regimes
than it is along cold fronts. So our highest chances of
lightning look to be earlier in the day Sunday, with a much
lower lightning threat with cold frontal convection, even if we
end up with a few hundred J/kg of SB CAPE. In the end, if a
locally damaging wind threat does materialize with the squall
line it is unlikely to be accompanied by lightning.

The 00z GFS and ECMWF both depict explosive deepening of the
surface cyclone Sunday afternoon and night, with both deepening
the surface low between 23-28mb in just 18 hours. This magnitude
of deepening is rare for this part country and in response, this
projected bomb cyclone will likely result in a large and very
powerful wind field Sunday night into Monday. Very strong cold
air advection will keep boundary layer well mixed overnight
Sunday and Monday, this should aid in transporting some powerful
gusts to the surface. Based on current model output, it is
possible that we could see winds flirting with high wind warning
criteria (sustained winds 40+ mph or gusts 58+ mph) for a short
time period late Sunday night/early Monday morning. It isn`t
uncommon for guidance to trend a bit weaker with high winds as
the event nears, so I bumped up winds and gusts solidly into
advisory criteria, but didn`t go as high as Bufkit momentum
transfer would suggest (at least not yet).

GFS and ECMWF both still suggest sufficient moisture wrapping
around the occluding cyclone could result in some wind driven
snow showers late Sunday night into Monday morning. Perhaps
enough to cover the ground, but confidence isn`t terribly high
in just how much QPF will be flung this far around the cyclone
into our CWA, so didn`t make any changes to the modest (30-60%)
POPs late Sunday night into Monday morning.

Temperatures behind this storm will only be slightly below
average, but the strong winds and dramatic contrast from the
unseasonably mild air preceding it, which will make the cold
all that much more biting. Winds will ease Monday night and
begin to back southwesterly Tuesday in advance of low pressure
progged to pass well to our north as it wraps around the
massive departing cyclone tracking north across Quebec. That
will be just one of what looks to be several vort lobes pivoting
around the huge, slow moving, closed mid-upper level low that
becomes established near Hudson Bay.

GFS and ECMWF both suggest a more potent/westward extending
shortwave could pivot around the upper low and dislodge a more
formidable shot of cold air New Year`s Eve night into New
Year`s Day. Still some variability in ensemble guidance with the
details on just how far west the coldest air gets and also
whether it will be accompanied by any snow. For now, made no
changes to NBM output, but this will be a time period we`ll need
to watch in coming days, particularly once we get past the
Sunday-Monday bomb cyclone.

- Izzi

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 542 AM CST Sat Dec 27 2025

Key Aviation Messages for the current TAF period:

- IFR conditions expected through the entire TAF window

- Drizzle/Showers move in tonight with steadier rainfall
  expected tomorrow morning with potential LIFR conditions
  during heavier precip after 12Z Sunday

Winds will be light and out of the southeast through the TAF
period. However, the main story today will be the low ceilings
and visibilities once again. Vis for areas away from the Chicago
Metro are currently between 3 to 4 SM, with a chance to lower
after day break down to around a mile with patchy fog possible.
Meanwhile, closer to the lake vis and cigs are slightly better,
but still with a chance to degrade in the next couple hours.
Dense fog is not expected at the Chicago Terminals with a less
than 20 percent chance for dense fog near KRFD.

There is moderate confidence in cigs and vis slightly lifting
back to higher end IFR after 16Z this morning. There are a few
models that show the slight chance for a brief window of low end
MVFR cigs this afternoon, but confidence in that transpiring is
low. There is another chance for lowering vis/fog after 00Z
tonight (with better chances near KRFD), not enough confidence
to mention it in the TAF presently. After 03Z tonight, a low
level jet will increase to around 40 knots out of the southwest
at or just above 2000 feet AGL. With light southeasterly winds
at the surface, there is a non-zero chance for low level wind
shear tonight, but seems both marginal and for a short enough
window to not need a formal mention of it in the TAF.

The next system will approach the airspace overnight tonight and
into tomorrow morning. Southerly flow will increase moisture for
more prevailing lower cigs and vis. Spotty light showers/drizzle
are expected around and after 06Z. Steadier rain can be expected
after 12Z wither moderate rain rates that can lower conditions
to LIFR levels. Lastly, the best chances for thunder with this
system looks to be far southwest of the Chicago Metro. While no
mention of thunder is needed at this window of the TAF, it will
be reviewed for later TAF packages.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Storm Watch from Sunday evening through Monday morning for
     Winthrop Harbor IL to Gary IN.

     Storm Watch from Sunday evening through Monday afternoon for
     Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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